1. Field of the Invention
The present invention relates to a magnetic field measuring apparatus using Superconducting Quantum Interference Device (SQUID) magnetometers (gradiometers) to sense extremely weak magnetic fields such as biomagnetic fields originating from a portion of a living body, e.g., heart, brain, etc., geomagnetism, and those involved in nondestructive testing, and particularly relates to such apparatus that achieves the cancellation of external magnetic noise that causes interference.
2. Description of Related Art
Magnetic field measuring apparatus using SQUID magnetometers (gradiometers) is applied to measurements of extremely weak magnetic fields in such a manner that magnetic fields in the brain, heart, etc. are measured in a magnetically shielded room featuring a rate of attenuating magnetic fields of 40 to 50 decibels (dB) or more. As a combination of pickup coils to sense biomagnetic fields, first-order gradiometers are often used that detect a differential signal occurring between a first one-turn pickup coil and a second coil wound in a direction opposite to the first coil. The first-order gradiometers are characterized in that they cancel external magnetic noise from a remote noise source, while can detect signals from a magnetic field generated in the vicinity of the object to be inspected, such as, heart, brain, etc., without causing significant cancellation, and therefore they can easily decrease the effect of the external magnetic noise. The first-order gradiometers generally attenuate uniform magnetic fields by about 40 to 50 dB.
The above-mentioned magnetically shielded room and gradiometers in combination can cancel external magnetic noise by about 80 to 100 dB or more. However, if a moving body such as an electric train or a motor vehicle passes rather near the magnetically shielded room, 50 to 100 meters away from the room, external magnetic noise several tens of times as large as a biomagnetic field originating from a living body may be observed. In order to cancel strong magnetic noise of this order, various methods have been attempted.
For example, a method has been proposed by which a flux-gate magnetometer is placed outside the magnetically shielded room and signals detected by it from a magnetic field are used to allow a feedback current to flow through compensating coils, or noise-canceling coils wound around the outside wall of the room, aiming at compensation to adjust the output of the flux-gate magnetometer theoretically to 0 (related art 1: Meas. Sci. Technol. Vol. 2, pp. 596-601 [1991]).
Another noise cancellation method by using SQUID sensors (high-order gradiometers) formed through software has been devised, wherein signals detected by reference sensor coils are used (related art 2: Clin. Phys. Physiol. Meas., Vol. 12, Suppl. B, pp. 81-86 [1991]). This method uses a low-order, hardware gradient detector which is then processed relative to a common reference system to form a higher-order gradient response, though the detail of the reference system is not stated.
Japanese Patent Prepublication No. Hei 11-47108 (related art 3) describes a biomagnetic field measuring apparatus for decreasing the error of environmental magnetic noise elimination due to the lost balance of the coils employed. According to this publication, the biomagnetic field measuring apparatus is summarized below. A plurality of coils that artificially generate an incoming magnetic field are installed in known position on, for example, the external spherical surface of a dewar. Driving current is allowed to flow through the coils to drive them. By using the values that the pickup coils detected as the strength of this magnetic field generated by the coils and the driving current value, the pickup coils sensitivities and their balance are measured with high accuracy. The biomagnetic measurements obtained from the values detected by the pickup coils are compensated with high accuracy for the effect of the lost coil balance. As a result, the error of environmental magnetic noise elimination caused by the lost balance of the coils is decreased and the accuracy of biomagnetic measurements is improved. In the vacuum adiabatic enclosure, a plurality of biomagnetic field measuring pickup coils that primarily measure biomagnetic fields are installed near the physical object to be inspected and a plurality of reference sensor coils that primarily measure environmental magnetic fields are installed farther than the pickup coils away from the object.
Japanese Patent Prepublication No. Hei 11-83965 (related art 4) describes a complex of an environmental noise canceling system and a magnetic measurement apparatus that enables highly precise measurement of extremely weak magnetic fields from the physical object of interest without being affected by environmental noise in different frequency bands. According to this publication, a plurality of SQUID gradiometers with different dynamic ranges and through rates sense different environmental magnetic fields with different field strengths in different frequency bands and measure electric signals corresponding to the sensed environmental magnetic fields. The noise canceling system can cancel noise of environmental magnetic fields with different field strengths in different frequency bands, by generating a magnetic field, through a pair of actively-shielded coils, in an opposite direction of environmental magnetic fields, based on addition electric signals obtained by adding all measured electric signals, or, by subtracting the addition electric signals from the magnetic signals measured by the SQUID gradiometers for magnetic measurement.
Japanese Patent Prepublication No. Hei 9-84777 (related art 5) describes a biomagnetic field measuring apparatus that can precisely probe a current dipole in deeper position among a plurality of current dipoles put together with one laid on top of another downward. According to this publication, the apparatus uses an array of pickup coils to detect magnetic flux originating from biomagnetic fields, the array formed by combining a plurality types of pickup coils that differ in gradient order or/and baseline.
Related art 1 aims at compensation to adjust the output of the flux-gate magnetometer placed outside the magnetically shielded room theoretically to 0 by flowing the feedback current through the compensating coils, or noise-canceling coils installed on the outside wall of the room. When this compensation method is applied to a multi-channel magnetic field measuring apparatus having SQUID magnetometers consisting of a plurality of differential-type pickup coils, or namely, first-order gradiometers, the coils of all channels may vary in the rate of noise cancellation. A problem arises that this variation of noise cancellation rate is difficult to rectify.
The document disclosing related art 2 simply describes the relevant data in a case where a magnetically shielded room is not used, but does not describe a specific configuration of the reference system using reference sensor coils.
An object of the present invention is to provide a magnetic field measuring apparatus that enables external magnetic noise cancellation with precision, allows for variation in the noise cancellation rate and baseline among the differential-type pickup coils employed to make the channels of a multi-channel magnetic field measuring apparatus when executing noise cancellation, and makes it possible to eliminate distorted magnetic signal waveforms induced by the frequency property of a magnetically shielded room in which the apparatus is installed.
Terminology used herein is explained below:
xe2x80x9cDifferential-type pickup coilsxe2x80x9d mean the coils constituting a first-order or second-order gradiometer.
(1) Providing differential type pickup coils constitute a first-order gradiometer:
(1.1) A xe2x80x9cgradiometer for detectionxe2x80x9d (first SQUID gradiometer) is a first-order SQUID gradiometer that detects the component in Z direction of magnetic noise from an external magnetic field as well as the component in Z direction of a biomagnetic field originating from a living body.
(1.2) A xe2x80x9cgradiometer for compensationxe2x80x9d (second SQUID gradiometer) is a first-order SQUID gradiometer that detects the component in Z direction of magnetic noise from an external magnetic field.
(1.3) An xe2x80x9cinput coil of a gradiometer for detectionxe2x80x9d means a first coil of the coils constituting a first-order gradiometer employed as one of the gradiometers for detection, positioned the nearest to the physical object to be inspected.
(1.4) An xe2x80x9cinput coil of a gradiometer for compensationxe2x80x9d means a first coil of the coils constituting a first-order gradiometer employed as one of the gradiometers for compensation, positioned the nearest to the physical object to be inspected.
(1.5) A xe2x80x9ccompensating coil of a gradiometer for detectionxe2x80x9d means a second coil whose base surface is parallel with the base surface of the first coil of the coils constituting a first-order gradiometer employed as one of the gradiometers for detection, positioned farther than the first coil away from the physical object to be inspected.
(1.6) A xe2x80x9ccompensating coil of a gradiometer for compensationxe2x80x9d means a second coil whose base surface is parallel with the base surface of the first coil of the coils constituting a first-order gradiometer employed as one of the gradiometers for compensation, positioned farther than the first coil away from the physical object to be inspected.
(1.7) xe2x80x9cOutputs of gradiometers for detectionxe2x80x9d mean the outputs from the first order gradiometers used as the gradiometers for detection.
(1.8) xe2x80x9cOutputs of gradiometers for compensationxe2x80x9d mean the outputs from the first order gradiometers used as the gradiometers for compensation.
(1.9) A xe2x80x9cbaseline of a gradiometer for detectionxe2x80x9d means the baseline of a first-order gradiometer employed as one of the gradiometers for detection and is a distance between the base surfaces of the first and second coils of the gradiometer.
(1.10) A xe2x80x9cbaseline of a gradiometer for compensationxe2x80x9d means the baseline of a first-order gradiometer employed as one of the gradiometers for compensation and is a distance between the base surfaces of the first and second coils of the gradiometer.
(2) Providing differential type pickup coils constitute a second-order gradiometer:
(2.1) A xe2x80x9cgradiometer for detectionxe2x80x9d (first SQUID gradiometer) is a second-order SQUID gradiometer that detects the component in Z direction of magnetic noise from an external magnetic field as well as the component in Z direction of a biomagnetic field originating from a living body.
(2.2) A xe2x80x9cgradiometer for compensationxe2x80x9d (second SQUID gradiometer) is a second-order SQUID gradiometer that detects the component in Z direction of magnetic noise from an external magnetic field.
(2.3) An xe2x80x9cinput coil of a gradiometer for detectionxe2x80x9d means a first coil of the coils constituting a second-order gradiometer employed as one of the gradiometers for detection, positioned the nearest to the physical object to be inspected.
(2.4) An xe2x80x9cinput coil of a gradiometer for compensationxe2x80x9d means a first coil of the coils constituting a second-order gradiometer employed as one of the gradiometers for compensation, positioned the nearest to the physical object to be inspected.
(2.5) xe2x80x9cCompensating coils of a gradiometer for detectionxe2x80x9d mean second, third, and fourth coils whose base surface is parallel with the base surface of the first coil of the coils constituting a second-order gradiometer employed as one of the gradiometers for detection, serially positioned in order farther than the first coil away from the physical object to be inspected. In the present invention, it is defined that the area of the second coil equals the area of the third coil.
(2.6) xe2x80x9cCompensating coils of a gradiometer for compensationxe2x80x9d mean second, third, and fourth coils whose base surface is parallel with the base surface of the first coil of the coils constituting a second-order gradiometer employed as one of the gradiometers for compensation, serially positioned in order farther than the first coil away from the physical object to be inspected. In the present invention, it is defined that the area of the second coil equals the area of the third coil.
(2.7) xe2x80x9cOutputs of gradiometers for detectionxe2x80x9d mean the outputs from the second-order gradiometers used as the gradiometers for detection.
(2.8) xe2x80x9cOutputs of gradiometers for compensationxe2x80x9d mean the outputs from the second-order gradiometers used as the gradiometers for compensation.
(2.9) xe2x80x9cBaselines of a gradiometer for detectionxe2x80x9d mean the baselines of a second-order gradiometer employed as one of the gradiometers for detection and are a distance between the base surfaces of the first and second coils and a distance between the base surfaces of the third and fourth coils of the gradiometer. In the present invention, it is defined that the distance between the base surfaces of the first and second coils equals the distance between the base surfaces of the third and fourth coils.
(2.10) xe2x80x9cBaselines of a gradiometer for compensationxe2x80x9d mean the baselines of a second-order gradiometer employed as one of the gradiometers for compensation and are a distance between the base surfaces of the first and second coils and a distance between the base surfaces of the third and fourth coils of the gradiometer. In the present invention, it is defined that the distance between the base surfaces of the first and second coils equals the distance between the base surfaces of the third and fourth coils.
A magnetic field measuring apparatus of the present invention is configured in hardware to primarily comprise a plurality of SQUID gradiometers that consist of differential-type pickup coils (first-order gradiometers or second-order gradiometers) and detect external magnetic noise as well as magnetic fields originating from a living body and a single unit or a plurality of units of SQUID gradiometers (second SQUID gradiometers) that also consist of differential-type pickup coils (first-order gradiometers or second-order gradiometers) and detect external magnetic noise. These SQUID gradiometers are arranged in a cryostat that is filled with cryogenic refrigerants such as liquid helium, liquid nitrogen, or the like; alternatively, a refrigerator is installed in the cryostat. A gantry supports the cryostat.
The apparatus is further configured such that the differential-type pickup coil baseline of the second SQUID gradiometer(s) is shorter than that of the first SQUID gradiometers, which thereby prevents the second gradiometer(s) from detecting the magnetic fields originating from a living body""s portion such as a heart.
By using mixed magnetic signals in which the external magnetic noise signals detected by the second SQUID gradiometer(s) are mixed with the biomagnetic signals and noise signals detected by the first SQUID gradiometers and applying the method of least squares, the apparatus computes an optimum fitting parameter and cancels (eliminates) the noise signals from the mixed magnetic signals detected (first method of canceling external magnetic noise).
The apparatus further eliminates the remaining external magnetic noise that it cannot cancel by the above first method of canceling external magnetic noise; in other words, the noise caused by the frequency property of a magnetically shielded room in which the apparatus is installed, through approximation of a theoretical waveform representing the noise whose magnetic field strength changes as a function of time, which is expressed by equation 1, and by calculating amplitude A and time constant T by the method of least squares.
B(t)=xe2x88x92Axc2x7t2xc2x7exp(xe2x88x92t/T)xe2x80x83xe2x80x83[Equation 1]
It should be noted that related arts 1, 2, 3, 4, and 5 disclose nothing corresponding to the features of the invention configured to: set the differential-type pickup coil baseline of the second SQUID gradiometer(s) for external magnetic noise detection shorter than that of the first SQUID gradiometers for biomagnetic detection; use a plurality of second SQUID gradiometers comprising differential-type pickup coils with different baselines; and cancel the external magnetic noise induced by the frequency property of the magnetically shielded room in which the apparatus is installed.
As a first configuration of the invention, a magnetic field measuring apparatus comprises a plurality of first SQUID gradiometers that detect signals in a normal direction of a magnetic field originating from a physical object to be inspected, a second SQUID gradiometer that detects signals in a normal direction of external magnetic noise, a cryogenic enclosure to keep the first and second SQUID gradiometers cold, a driving circuit to drive the first and second SQUID gradiometers, a computer that executes signal processing after collecting the signals detected by the first and second SQUID gradiometers, and a display means for displaying the results of signal processing. The magnetic field measuring apparatus is characterized in that the first and second SQUID gradiometers comprise differential-type pickup coils (to constitute first-order or second-order gradiometers), some of which are used as compensating coils, and the length of differential-type pickup coil baseline of second SQUID gradiometer is shorter than the length of differential-type pickup coil baselines of the first SQUID gradiometers.
In the first configuration of the invention, the magnetic field measuring apparatus is further characterized as follow.
(1) The apparatus placed in a magnetically shielded room detects the signals in a normal direction of a magnetic field originating from the physical object to be inspected and includes a signal unit or a plurality of units of second SQUID gradiometers. The computer executes first signal processing by using mixed magnetic signals in which the external magnetic noise signals detected by the second SQUID gradiometer(s) are mixed with biomagnetic signals and external magnetic noise signals detected by the first SQUID gradiometers and applying the method of least squares, thus canceling the external magnetic noise from the mixed magnetic signals. Moreover, the computer executes second signal processing in such a manner that the computer executes approximation of waveform B(t), which represents the waveform of external magnetic noise occurring due to the frequency property of the magnetically shielded room near the initial time at which such noise begins to occur, by using equation B(t)=xe2x88x92Axc2x7t2xc2x7exp (xe2x88x92t/T), where A is amplitude, T is time constant, and t is time variable, calculates amplitude A and time constant T, according to the method of least squares, by using the magnetic signal waveform obtained through the first processing, and cancels such noise from the magnetic signal waveform obtained through the first signal processing by using the waveform B(t) determined by the method of least squares.
(2) The computer infers the initial time and the inferred initial time is indicated on the time axis for showing a magnetic signal waveform from which the external magnetic noise has been canceled by the first and second signal processing.
(3) The input coil area of the differential-type pickup coils of the second SQUID gradiometer(s) is greater than the input coil area of the differential-type pickup coils of the first SQUID gradiometers.
As a second configuration of the invention, a magnetic field measuring apparatus comprises a plurality of first SQUID gradiometers that detect signals in a normal direction of a magnetic field originating from a physical object to be inspected within a magnetically shielded room, a second SQUID gradiometer that detects signals in a normal direction of external magnetic noise, a cryogenic enclosure to keep the first and second SQUID gradiometers cold, a driving circuit to drive the first and second SQUID gradiometers, a computer that executes signal processing after collecting the signals detected by the first and second SQUID gradiometers, and a display means for displaying the results of signal processing. The magnetic field measuring apparatus is characterized in that the first and second SQUID gradiometers comprise differential-type pickup coils (to constitute first-order or second-order gradiometers), some of which are used as compensating coils, the baseline length of the differential-type pickup coils of the second SQUID gradiometer is shorter than the baseline length of the differential-type pickup coils of the first SQUID gradiometers, and the computer executes signal processing (a) to cancel the external magnetic noise due to variant noise cancellation rates of the first SQUID gradiometers from the magnetic signal waveform in a normal direction obtained through detection by the first SQUID gradiometers and signal processing (b) to cancel the external magnetic noise occurring due to the frequency property of the magnetically shielded room from the magnetic signal waveform obtained through the signal processing (a).
In the second configuration of the invention, the magnetic field measuring-apparatus is further characterized in that:
(1) the computer infers initial time at which external magnetic noise begins to occur due to the frequency property of the magnetically shielded room and the inferred initial time is indicated on the time axis for showing a magnetic signal waveform from which the external magnetic noise has been canceled by signal processing (a) and (b); and
(2) the input coil area of the differential-type pickup coils of the second SQUID gradiometer is greater than the input coil area of the differential-type pickup coils of the first SQUID gradiometers.
As a third configuration of the invention, a magnetic field measuring apparatus comprises a plurality of first SQUID gradiometers that detect signals in a normal direction of a magnetic field originating from a physical object to be inspected within a magnetically shielded room, second and third SQUID gradiometers that detect signals in a normal direction of external magnetic noise, a cryogenic enclosure to keep the first, second, and third SQUID gradiometers cold, a driving circuit to drive the first, second, and third SQUID gradiometers, a computer that executes signal processing after collecting the signals detected by the first, second, and third SQUID gradiometers, and a display means for displaying the results of signal processing. The magnetic field measuring apparatus is characterized in that the first, second, and third SQUID gradiometers comprise differential-type pickup coils (to constitute first-order or second-order gradiometers), some of which are used as compensating coils, the baseline lengths of the differential-type pickup coils of the second and third SQUID gradiometers are shorter than the baseline length of the differential-type pickup coils of the first SQUID gradiometers, the baseline length of the differential-type pickup coils of the second SQUID gradiometer is shorter than the baseline length of the differential-type pickup coils of the third SQUID gradiometer, and the computer executes signal processing (a) to cancel the external magnetic noise due to variant noise cancellation rates of the first SQUID gradiometers from the magnetic signal waveform in a normal direction obtained through detection by the first SQUID gradiometers, signal processing (b) to cancel the external magnetic noise due to that the baseline of the differential-type pickup coils of the second SQUID gradiometer differs from the baseline of the differential-type pickup coils of the third SQUID gradiometer from the magnetic signal waveform obtained through the signal processing (a), and signal processing (c) to cancel the external magnetic noise occurring due to the frequency property of the magnetically shielded room from the magnetic signal waveform obtained through the signal processing (b).
In the third configuration of the invention, the magnetic field measuring apparatus is further characterized in that:
(1) the computer infers initial time at which external magnetic noise begins to occur due to the frequency property of the magnetically shielded room and the inferred initial time is indicated on the time axis for showing a magnetic signal waveform from which the external magnetic noise has been canceled by signal processing (a), (b), and (c); and
(2) the input coil area of the differential-type pickup coils of the second and third SQUID gradiometers is greater than the input coil area of the differential-type pickup coils of the first SQUID gradiometers.
As a fourth configuration of the invention, a magnetic field measuring apparatus comprises a plurality of first SQUID gradiometers that detect signals in a normal direction of a magnetic field originating from a physical object to be inspected within a magnetically shielded room which blocks out high-frequency electromagnetic waves, a plurality of second SQUID gradiometers that detect signals in a normal direction of external magnetic noise, a cryogenic enclosure to keep the first and second SQUID gradiometers cold, a driving circuit to drive the first and second SQUID gradiometers, noise-canceling coils positioned over and under the physical object to be inspected to generate a normal-directional magnetic field that cancels the external magnetic noise, a control means for controlling the current flowing through the noise-canceling coils, and a computer that executes signal processing after collecting the signals detected by the first and second SQUID gradiometers. The magnetic field measuring apparatus is characterized in that the first and second SQUID gradiometers comprise differential-type pickup coils (to constitute first-order or second-order gradiometers), some of which are used as compensating coils, the baseline length of the differential-type pickup coils of the second SQUID gradiometers is shorter than the baseline length of the differential-type pickup coils of the first SQUID gradiometers, the control means controls the current flowing through the noise-canceling coils on the basis of the output from one of the second SQUID gradiometers, and the noise-canceling coils generate a normal-directional magnetic field that exerts force in a direction opposite to the external magnetic noise.
In the fourth configuration of the invention, the magnetic field measuring apparatus is further characterized in that:
(1) the input coil area of the differential-type pickup coils of the second SQUID gradiometers is greater than the input coil area of the differential-type pickup coils of the first SQUID gradiometers.
(2) a separate driving circuit is provided to drive one of the second SQUID gradiometers and the noise-canceling coils also serve as feedback coils to be used in the separate drive circuit.
(3) the noise-canceling coils are placed inside or outside the magnetically shielded room.
As a fifth configuration of the invention, a magnetic field measuring apparatus comprises a plurality of first SQUID gradiometers that detect signals in a predetermined direction or a combination of predetermined directions of a magnetic field originating from a physical object to be inspected, a second SQUID gradiometer that detects signals in a predetermined direction or a combination of predetermined directions of external magnetic noise, a cryogenic enclosure to keep the first and second SQUID gradiometers cold, a driving circuit to drive the first and second SQUID gradiometers, a computer that executes signal processing after collecting the signals detected by the first and second SQUID gradiometers, and a display means for displaying the results of signal processing. The magnetic field measuring apparatus is characterized in that the first and second SQUID gradiometers comprise differential-type pickup coils and the baseline length of the differential-type pickup coils of the second SQUID gradiometer is shorter than the baseline length of the differential-type pickup coils of the first SQUID gradiometers.
In the fifth configuration of the invention, a predetermined direction or a combination of predetermined directions is defined as at least any one of the x, y, and z directions. Besides, the predetermined direction or the combination of predetermined directions includes a normal direction or/and a tangential direction. The apparatus in the fifth configuration detects signals of normal or/and tangential directional components of a magnetic field originating from the object to be inspected and executes signal processing to cancel the external magnetic noise in normal or/and tangential directions.
With reference to FIG. 1, a typical configuration of the invention is summarized below. The apparatus configured as in FIG. 1 primarily comprises a plurality of first SQUID gradiometers 9 that detect signals of a biomagnetic field and external magnetic noise, a plurality of second SQUID gradiometers 10 that detect signals of external magnetic noise, a driving circuit 6 to drive the first and second SQUID gradiometers, a computer 8 that executes signal processing after collecting the signals detected by the first and second SQUID gradiometers. The first and second SQUID gradiometers comprise differential-type pickup coils to constitute first-order gradiometers. The baselines of the coils of first-order gradiometers of the second SQUID gradiometers are shorter than that of the first SQUID gradiometers. From the detected biomagnetic signal waveforms, the apparatus executes processing to cancel external magnetic noise that exists there due to the following: the coils of first-order gradiometers of the first SQUID gradiometers give variant noise cancellation rates; the baselines formed by the coils of the first-order gradiometers of the second SQUID gradiometers differ, and noise is induced by the frequency properties of the magnetically shielded room in which the apparatus is installed. The invention in this configuration can provide a magnetic field measuring device that can exactly cancel even spiky and considerably strong environmental noise.
According to the invention that can be configured in several ways described above, the SQUID gradiometers for detecting signals in a normal direction of a magnetic field originating from a living body detects biomagnetic signals into which some external magnetic noise in a normal direction is mixed, and from which normal-directional biomagnetic signals can be separated and extracted through the use of the normal-directional external magnetic noise signals detected by the SQUID gradiometer(s) for detecting external magnetic noise.
Furthermore, distorted magnetic signal waveforms in a normal direction, induced by the frequency property of the magnetically shielded room in which the apparatus is installed can be eliminated by using a theoretical equation.
According to the invention that can be configured in several ways, a biomagnetic field measuring apparatus can be made to enable sensitive measurement of an extremely weak magnetic field originating from the physical object to be inspected, for example, a human""s heart, even an unborn baby""s heart in the mother""s body.
According to the invention, variant noise cancellation rates for uniform magnetic fields of differential-type pickup coils to constitute SQUID gradiometers and different baselines formed by these coils are taken into consideration when external magnetic noise is canceled, and therefore noise can be canceled precisely.
A magnetic field measuring apparatus offered by the invention can exactly execute the cancellation of external magnetic noise due to the frequency property of a magnetically shielded room in which the apparatus is installed and can precisely cancel even spiky and considerably strong environmental noise.
Furthermore, the invention can offer a biomagnetic field measuring apparatus enabling sensitive measurement of an extremely weak magnetic field originating from a physical object to be inspected, for example, a human""s heart, even an unborn baby""s heart in the mother""s body.